1. Field of the Invention
The present invention relates to a unit type linear scale having a detection head integrally formed with a frame body housing a plate-shaped main scale, wherein the detection head detects the amount of movement in a direction of length measurement based on the amount of relative movement of an index scale that moves along the main scale.
2. Description of the Related Art
A commonly used unit type linear scale is such that a long length of a frame body housing a main scale is fixed to one (object of measurement) of moving bodies that move relatively to each other, a detection head housing an index scale is fixed to the other moving body, and the amount of relative movement of the moving bodies is detected in length measurement by reading the main scale with the detection head. In this unit type linear scale, the detection head is integrally attached to the frame body and movable in a direction of length measurement.
Such a unit type linear scale includes, as means for fixing the frame body in which a main scale is housed on an object (moving body) of measurement, a multi-position fixing method with which the frame body is directly fixed with screws at a plurality of positions along the direction of length measurement.
FIG. 3A is a perspective view showing an aluminum frame (frame body) 10 of this unit type linear scale with the multi-position fixing method fixed to a fixed object (not shown) of measurement, and FIG. 3B shows a side cross sectional view of the aluminum frame.
As shown in the figures, the aluminum frame 10 is formed in a hollow shape extending in the direction of length measurement with an opening 12 at the lower part thereof, and a main scale 14 is secured on the wall surface of one side of a groove 16, formed inside the aluminum frame 10, by being pressed by a round cross section rubber piece (elastic member) 18 provided in the groove 16.
Conventionally, the main scale 14 has been fixed to the aluminum frame 10 in such a way that the entire part of the main scale 14 in the length direction (length measurement direction) is pressed by the round cross section rubber piece 18, and additionally, bonded with a silicon adhesive 20.
In the scale unit 10, a detection portion such as an index scale included in a detection head (not shown) is extended from the opening 12, and the detection portion is movable along the main scale 14 in the direction of length measurement.
A unit type linear scale with the multi-position fixing method as described above is excellent in vibration resistance characteristics, because, as shown in the figure, the frame body is directly fixed to an object of measurement with screws 22 at a plurality of positions (four positions in this case) along the direction of length measurement of the scale unit 10.
However, a problem in a unit type linear scale with the multi-position fixing method is that measurement accuracy is lowered by a wave form of the aluminum frame 10 and main scale 14 in the direction of length measurement caused by tightening force by means of the screws applied when the aluminum frame 10 is fixed. It is also a problem that measurement accuracy is also lowered by the unstable behavior of elongation of the main scale 14 against change in temperature, which occurs because the round cross section rubber piece 18 is used for the entire length of the main scale 14 and aluminum frame 10, and friction force between the main scale 14 and the aluminum frame 10 widely varies owing to varied characteristics of the round cross section rubber piece 18 such as its pressing force and dimensions.
It is an object of the present invention to provide a unit type linear scale with a multi-position fixing method that can improve measurement accuracy without deteriorating the vibration resistance characteristics.
The present invention is intended to solve the problems described above by providing a unit type linear scale having a frame body that houses a plate-shaped main scale and extends in a direction of length measurement. The frame body is fixed with screws at multiple positions to an object of measurement through screw holes formed at a plurality of positions along the direction of length measurement. In the scale, one surface of the main scale is made to contact with the wall surface of one side of a groove which is formed in the frame and extended in the direction of length measurement. An elastic member is provided between, however excluding portions where the screw holes are provided, the wall surface of the groove on the other side of the groove along the direction of length measurement and the other surface of the main scale, and thereby the scale is secured on the frame.
Namely, in the present invention, it is arranged in such a way that an elastic member for securing the main scale on the frame body is not provided at portions where screw holes are formed and strong tightening force is applied by screws when fixing the frame body to an object of measurement. Measurement accuracy can be improved when compared with a case where an elastic member is provided along the entire length of the main scale, because the transmission of a wave produced in the frame body to the main scale is prevented even when the frame body is screwed to an object of measurement.
Therefore, measurement accuracy is improved in a unit type linear scale without deteriorating vibration resistance characteristics.